Uptake and degradation of rat chylomicron remnants, produced in vivo and in vitro, in rat hepatocyte monolayers.

1. The uptake of small and large chylomicrons in rat hepatocyte monolayer cultures was compared to the uptake of chylomicron remnants prepared either in vitro with pure milk lipoprotein lipase or in hepatectomized rats. 2. Small chylomicrons (Sf less than 400) markedly inhibited remnant uptake and were taken up more efficiently than large ones (Sf greater than 400), indicating that size may be an important factor for the rate of uptake. The Lineweaver-Burk analysis of the data indicated that the V values for the uptake of both small chylomicrons (Sf less than 400) and of remnants prepared either in hepatectomized rats or in vitro was significantly higher than for chylomicrons with Sf greater than 400, whereas the Km values for the different particles did not differ significantly. 3. Preincubation of chylomicrons with serum caused marked changes in their apolipoprotein composition. A loss of apolipoprotein A-I and an increase in apolipoprotein E content was observed by scanning of SDS-polyacrylamide gels. Th preincubation decreased, however, the subsequent uptake of the chylomicrons. In contrast, the uptake of remnants prepared in vivo, or in vitro with serum present, exceeded that of remnants prepared in vitro with albumin or fetal calf serum as the fatty acid acceptor. 4. The data thus indicate that both the decrease in size and the changes in the particle surface during lipolysis with serum present are likely to contribute to the differences seen in the rate of uptake between native chylomicrons and remnants in hepatocyte monolayers.